The present invention relates to a liquid-crystalline medium based on a mixture of polar compounds of negative dielectric anisotropy, and to the use thereof for electro-optical purposes and to displays containing this medium, in particular displays with active-matrix addressing based on the ECB effect and especially on the vertically aligned (VA) effect.
Matrix liquid-crystal (MLC)displays of this type are known. Non-linear elements which can be used for individual switching of the individual pixels are, for example, active elements (i.e., transistors). The term xe2x80x9cactive matrixxe2x80x9d is then used, where a distinction can be made between two types.
1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer as substrate.
2. Thin-film transistors (TFTs) on a glass plate as substrate.
The use of single-crystal silicon as substrate material restricts the display size, since even modular assembly of various part-displays results in problems at the joints, which is why type 2 is preferred. A distinction is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. TFT displays usually operate as TN cells with crossed polarisers in transmission and are back-lit.
The term MLC displays here covers any matrix display with integrated non-linear elements, i.e., besides the active matrix, also displays with passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
In liquid-crystal displays of this type, the liquid crystals are used as dielectrics, whose optical properties change reversibly on application of an electric voltage. Electro-optical displays which use liquid crystals as media are known to the person skilled in the art. These liquid-crystal displays use various electro-optical effects.
The principle of electrically controlled birefringence, the ECB (xe2x80x9celectrically controlled birefringencexe2x80x9d) effect or DAP (xe2x80x9cdeformation of aligned phasesxe2x80x9d) effect, was described for the first time in 1971 (M. F. Schieckel and K. Fahrenschon, xe2x80x9cDeformation of nematic liquid crystals with vertical orientation in electrical fieldsxe2x80x9d, Appl. Phys. Lett. 19 (1971), 3912). This was followed by papers by J. F. Kahn (Appl. Phys. Left. 20 (1972), 1193) and G. Labrunie and J. Robert (J. Appl. Phys. 44 (1973), 4869).
The papers by J. Robert and F. Clerc (SID 80 Digest Techn. Papers (1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82 Digest Techn. Papers (1982), 244) have shown that liquid-crystalline phases must have high values for the ratio of the elastic constants K33/K11, high values for the optical anisotropy xcex94n, and values for the dielectric anisotropy xcex94xcex5 of from about xe2x88x920.5 to about xe2x88x925 in order to be suitable for high-information display elements based on the ECB effect. Electro-optical display elements based on the ECB effect have a homeotropic edge alignment.
However, liquid-crystal displays of this type have some disadvantages compared with the known active-matrix TN displays, in particular a high viewing-angle dependence of the contrast ratio and of the grey shades.
A more recent variant of the ECB displays is the active matrix display based on the VAN (vertically aligned nematic) effect and the VAC (vertically aligned cholesteric) effect. VAN displays have been described, inter alia, in S. Yamauchi et al., SID Digest of Technical Papers, pp. 378 ff (1989), and VAC displays have been described in K. A. Crabdall et al., Appl.Phys.Lett. 65, 4 (1994).
The more recent VAN and VAC displays, like the ECB displays already disclosed earlier, contain a layer of a liquid-crystalline medium between two transparent electrodes, the liquid-crystal medium having a negative value for the dielectric constant anisotropy xcex94xcex5. The molecules of this liquid-crystal layer have a homeotropic or tilted homeotropic alignment (i.e. substantially perpendicular to the electrode surfaces) in the switched-off state. Owing to the negative xcex94xcex5, realignment of the liquid-crystal molecular parallel to the electrode surfaces takes place in the switched-on state.
In contrast to conventional ECB displays, in which the liquid-crystal molecules have, in the switched-on state, a parallel alignment with a preferential direction which is uniform over the entire liquid-crystal cell, in VAN and VAC displays this uniform parallel alignment is restricted only to small domains within the cell. Disclinations exist between these domains, also known as tilt domains.
As a consequence of this, VAN and VAC displays have greater viewing-angle independence of the contrast and of the grey shades compared with conventional ECB displays. In addition, displays of this type are simpler to produce since additional treatment of the electrode surface for uniform alignment of the molecules in the switched-on state, such as, for example, by rubbing, is no longer necessary.
In contrast to VAN displays, the liquid-crystal media in VAC displays additionally comprise one or more chiral compounds, such as, for example, chiral dopants, which, in the switched-on state, induce a helical twist of the liquid-molecules in the liquid-crystal layer by an angle of between 0 and 360xc2x0. The twist angle in the preferred case is about 90xc2x0.
Also known are liquid-crystal display elements using the IPS effect (in plane switching), in which both dielectrically positive and dielectrically negative liquid-crystal media can be used. Likewise, the dyes in guest/host displays can be employed either in dielectrically positive or dielectrically negative media, depending on the display mode used.
A further type of liquid-crystal display in which dielectrically negative media are used are the so-called xe2x80x9caxially symmetric microdomainxe2x80x9d (ASM for short) displays, which are preferably addressed by means of plasma arrays (PALCDs, from xe2x80x9cplasma-addressed liquid-crystal displaysxe2x80x9d).
The above-mentioned display elements, in particular those which operate on the VA effect, generally have relatively short response times. However, there is an increasing demand, in particular in TV and video applications, for displays having even shorter response times. This can in principle be achieved either by reducing the rotational viscosities or by reducing the layer thickness d in the display elements. In order to keep the dxc2x7xcex94n value in the required range, liquid-crystal media having higher values of the optical anisotropy xcex94n are therefore necessary for display elements having smaller layer thicknesses d.
In addition, the chemical resistance to moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet region, as well as direct and alternating electric fields is important. Furthermore, LC (liquid crystal) phases which can be used industrially are required to have a liquid-crystalline mesophase in a suitable temperature range, low viscosity and the highest possible value for the voltage holding ratio.
An object of the invention is to provide a liquid-crystalline medium based on a mixture of polar compounds of negative dielectric anisotropy which at least substantially meets the above-mentioned requirements, in particular has low rotational viscosities and/or comparatively high values of the optical anisotropy xcex94n.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that these objects can be achieved if media according to the invention are used in displays.
The invention thus relates to a liquid-crystalline medium based on a mixture of polar compounds of negative dielectric anisotropy, characterised in that it comprises one or more compounds of the general formula I 
in which
R11 is an alkyl group having from 1 to 12 carbon atoms or an alkenyl group having from 2 to 12 carbon atoms, and
R12 is an alkenyl group having from 2 to 12 carbon atoms.
Compounds of the formula I in which R11 is C1-10-alkyl and R12 is 3-butenyl are described in JP 09208503 A as components of liquid-crystalline mixtures having only positive values of the dielectric anisotropy.
Preferably the concentration of dielectrically positive compounds in the mixture is 0% to 25%, more preferably 0% to 10%, and in particular 5% or less. In a preferred embodiment the mixtures do not contain any dielectrically positive compounds at all, at least only less than or equal to 1%.
Preferably the polar compounds of the mixture comprise one or more compounds with a terminal fluorine and/or one or more compounds with a fluorinated terminal group, e.g., OCF3 or OCHF2.
It has now been found, entirely unexpectedly, that the compounds of the formula I are very advantageously suitable as components in liquid-crystalline media based on a mixture of polar compounds of negative dielectric anisotropy. Thus, the use of one or more compounds of the formula I enables comparatively high values of the optical anisotropy xcex94n to be achieved without the rotational viscosity values being impaired. In addition, it is generally possible to achieve even lower rotational viscosities.
Furthermore, use of one or more compounds of the formula I enables mixtures having a broad liquid-crystalline mesophase range in a suitable temperature range, low viscosity and comparatively high values for the voltage holding ratio (HR) to be achieved.
The compounds of the formula I themselves are stable to moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet region, as well as direct and alternating electric fields.
The media according to the invention exhibit very high HR values, low threshold voltages and in particular very good low-temperature stabilities at the same time as high clearing points.
The liquid-crystalline media according to the invention are advantageously suitable for use in liquid-crystal matrix display elements in which liquid-crystal mixtures of negative dielectric anisotropy (xcex94xcex5 less than 0) are employed. These are, in particular, display elements which operate on the principles mentioned at the outset, such as the ECB and VA principles, including vertically aligned nematics (VAN), vertically aligned cholesterics (VAC), multi-domain vertically aligned (MVA), and patterned vertically aligned (PVA) modes. Furthermore, the media according to the invention can also advantageously be employed in ASM and PALC displays and displays operating on the IPS effect. Their use is also possible in guest/host displays. Preference is given here to MLC displays having an actively addressed matrix, in particular TFT displays.
The invention therefore furthermore relates to an electro-optical display element which contains, as dielectric, a liquid-crystalline medium according to the invention. Preference is given to those which operate in accordance with the ECB principle, in particular the VA principle. Actively addressed display elements are particularly advantageous here.
Preferred embodiments are given below:
The meaning of R11 in the formula I includes straight-chain and branched alkyl having from 1 to 12 carbon atoms, preferably having from 1 to 7 carbon atoms. The alkyl radical is preferably straight-chain, and R11 is therefore preferably methyl, ethyl, propyl, butyl, pentyl, hexyl or heptyl. R11 may furthermore also be octyl, nonyl, decyl, undecyl or dodecyl. R11 is very particularly preferably methyl, ethyl, propyl, butyl or pentyl, in particular methyl.
The meaning of R12 and/or R11 in the formula I includes straight-chain and branched alkenyl having from 2 to 12 carbon atoms, preferably having from 2 to 5 carbon atoms. Straight-chain alkenyl groups are preferred. Preference is furthermore given to C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl. Particularly preferred meanings of R12 are vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl or oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl. Of these, the 3-alkenyls, in particular but-3-enyl and pent-3-enyl, are very particularly preferred.
Very particularly preferred liquid-crystal mixtures according to the invention therefore comprise one or more compounds of the formulae Ia, Ib, Ic and/or Id 
in which R11 is as defined above, in particular is alkyl, preferably methyl, ethyl, propyl, butyl or pentyl, particularly preferably methyl.
Further preferred embodiments are given below:
a) Medium which additionally comprises one or more compounds of the formula II: 
in which
m is 1 or 2,
Z21 is xe2x80x94COOxe2x80x94 or a single bond, and
A21 is trans-1,4-cyclohexylene or 1,4-phenylene,
R21 and R22, independently of one another, are an alkyl or alkenyl group having up to 12 carbon atoms, in which, in addition, one or more non-adjacent CH2 groups may be replaced in each case by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94.
b) Medium which additionally comprises one or more compounds of the formula III: 
in which
m is 1 or 2,
A is trans-1,4-cyclohexylene or 1,4-phenylene, in which one H atom may be replaced by F,
R31 and R32, independently of one another, are an alkyl or alkenyl group having up to 12 carbon atoms, in which, in addition, one or more non-adjacent CH2 groups may be replaced in each case by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94.
c) Medium which additionally comprises one or more compounds of the formula IV 
in which R41 and R42, independently of one another, are as defined for R22.
d) Medium which additionally comprises one or more compounds of the formula VI 
in which R61 and R62, independently of one another, are as defined for R22, and 
independently of one another, are 
preferably, independently of one another, are 
one of
Z61 and Z62 is OCF2 or CF2O and the other is a single bond, and
n is 0 or 1.
e) Medium which additionally comprises one or more compounds of the formula VII 
in which
R71 and R72 are each, independently of one another, as defined for R22,
Z71 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COOxe2x80x94 or a single bond, 
are each, independently of one another, 
f) Medium which comprises one or more compounds selected from the formulae IIa to IIj: 
in which alkyl is in each case, independently of one another, a straight-chain alkyl group having from 1 to 6 carbon atoms, n is 0 or 1, and each alkenyl is a straight-chain alkenyl group having from 2 to 6 carbon atoms. Particular preference is given here to a medium having one or more compounds of the formulae IIa, IIb, IIc, IIg, IIi and/or IIj. In accordance with the invention, low rotational viscosities can be achieved, in particular, using media comprising compounds of the formula IIg. In addition, compounds of the formula IIi, in particular where n=1, can advantageously be employed in media of high optical anisotropy, in particular xcex94n greater than 0.11.
g) Medium which comprises one or more compounds selected from the formulae IIIa to IIIg: 
in which alkyl is in each case, independently of one another, a straight-chain alkyl group having from 1 to 6 carbon atoms, alkenyl is a straight-chain alkenyl group having from 2 to 6 carbon atoms, n is 0 or 1, and L is H or F. Particular preference is given here to a medium having one or more compounds of the formulae IIIb, IIIc, IIIf and/or IIIg. In accordance with the invention, low rotational viscosities can be achieved, in particular, using media comprising compounds of the formula IIIc.
h) Medium which comprises one or more compounds selected from the formulae VIa to VIq: 
in which alkyl is in each case, independently of one another, a straight-chain alkyl group having from 1 to 6 carbon atoms, and n is 0 or 1.
i) Medium which comprises one or more compounds selected from the formulae VIIa to VIId: 
in which R71 and R72 are each as defined above under the formula
VII. R71 is preferably n-alkyl having from 1 to 5 carbon atoms, particularly preferably having from 1 to 3 carbon atoms, and R72 is preferably n-alkyl or n-alkoxy having from 1 to 5 carbon atoms or alkenyl having from 2 to 5 carbon atoms.
k) Medium which comprises 1, 2 or 3 compounds of the formula I.
l) Medium which comprises at least one compound of the formula I and at least one compound of the formulae IId, IIf and/or IIIc.
m) Medium which consists essentially of at least one compound of the formula I, at least one compound of the formula II and at least one compound of the formula III.
n) Medium which consists essentially of at least one compound of the formula I, at least one compound of the formula II and at least one compound of the formula VI.
o) Medium which consists essentially of at least one compound of the formula I, at least one compound of the formula II and at least one compound of the formula VII.
p) Medium which comprises 2-35% by weight, preferably 2-25% by weight, particularly preferably 4-18% by weight, of one or more compounds of the formula I.
q) Medium which comprises 20-90% by weight, preferably 30-85% by weight, particularly preferably 40-80% by weight, of one or more compounds of the formula II.
r) Medium which comprises 5-60% by weight, preferably 10-40% by weight, of one or more compounds of the formula III.
s) Medium comprising
2-25% by weight of one or more compounds of the formula I,
30-85% by weight of one or more compounds of the formula II, and
10-40% by weight of one or more compounds of the formula III.
The liquid-crystal mixture preferably has a nematic phase range of at least 80 K, particularly preferably of at least 100 K, and a rotational viscosity of not greater than 300 mPaxc2x7s, in particular less than or equal to 250 mPaxc2x7s and particularly preferably not greater than 200 mPaxc2x7s, at 20xc2x0 C.
The liquid-crystal mixture according to the invention has a dielectric anisotropy xcex94xcex5 of preferably less than or equal to xe2x88x920.5, more preferably less than or equal to xe2x88x922.0, particularly preferably less than or equal to xe2x88x923.0. A preferred range of values for xcex94xcex5 is from about xe2x88x920.5 to xe2x88x928, in particular from about xe2x88x922.0 to xe2x88x927.0, particularly preferably from about xe2x88x923.0 to xe2x88x925.5, in each case determined at 20xc2x0 C. and 1 kHz. The dielectric constant xcex5∥ is generally greater than or equal to 3, preferably from 3.2 to 4.5.
The birefringence xcex94n in the liquid-crystal mixture generally has a value greater than 0.060, preferably greater than or equal to 0.075, particularly preferably greater than or equal to 0.090. Media of this type having comparatively high optical anisotropy generally have xcex94n values of up to 0.14, with xcex94n values of up to 0.17 or higher also being conceivable.
The dielectrics may also comprise further additives known to the person skilled in the art and described in the literature. For example, 0-15% by weight of pleochroic dyes may be added, furthermore conductive salts, preferably ethyldimethyldodecylammonium 4-hexyloxybenzoate, tetrabutyl-ammonium tetraphenylboranate or complex salts of crown ethers (cf., for example, Haller et al., Mol. Cryst. Liq. Cryst. 24, 249-258 (1973)) may be added in order to improve the conductivity, or substances may be added in order to modify the dielectric anisotropy, the viscosity and/or the alignment of the nematic phases. Substances of this type are described, for example, in DE-A 22 09 127, 22 40 864, 23 21 632, 23 38 281, 24 50 088, 26 37 430 and 28 53 728.
The individual components of the above-mentioned formulae of the liquid-crystal mixtures according to the invention are either known or their methods of preparation can easily be derived from the prior art by the person skilled in the relevant art since they are based on standard methods described in the literature.
The term xe2x80x9calkylxe2x80x9d in formulae II to VII includes straight-chain and branched alkyl having up to 12 carbon atoms, preferably from 1 to 7 carbon atoms, and is therefore, in particular, methyl, ethyl, propyl, butyl, pentyl, hexyl or pentyl. The meanings octyl, nonyl, decyl, undecyl and dodecyl are also possible.
The term xe2x80x9calkenylxe2x80x9d in formulae II to VII includes straight-chain and branched alkenyl having up to 12, preferably having from 2 to 7, carbon atoms. Straight-chain alkenyl groups are preferred. Further preferred are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4alkenyl.
Of these groups, particular preference is given to vinyl, 1 E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl and 6-heptenyl. alkenyl groups having up to 5 carbon atoms are particularly preferred.
The nematic liquid-crystal mixtures in the displays according to the invention generally comprise two components A and B, which themselves consist of one or more individual compounds.
Component A has significantly negative dielectric anisotropy and gives the nematic phase a dielectric anisotropy of xe2x89xa6xe2x88x920.3. It preferably comprises compounds of the formulae II and/or VI.
The proportion of component A is preferably between 45 and 100% by weight, in particular between 60 and 100% by weight.
For component A, one or more individual compounds which have a value of xcex94xcex5 of xe2x89xa6xe2x88x920.8 are preferably selected. This value must be more negative the smaller the proportion of component A in the mixture as a whole.
Particularly preferred individual compounds of component B are extremely low-viscosity viscous nematic liquid crystals having a viscosity of not greater than 18 mm2xc2x7sxe2x88x921, preferably not greater than 12 mm2xc2x7sxe2x88x921 at 20xc2x0 C.
Component B is monotropically or enantiotropically nematic, has no smectic phases and is able to prevent the occurrence of smectic phases down to very low temperatures in liquid-crystal mixtures. For example, if various materials of high nematogeneity are added to a smectic liquid-crystal mixture, the nematogeneity of these materials can be compared through the degree of suppression of smectic phases that is achieved. A multiplicity of suitable materials is known to the person skilled in the art from the literature. Particular preference is given to compounds of the formula II.
The liquid-crystal mixtures according to the invention preferably comprise from 4 to 25, in particular from 6 to 18, compounds of the formulae I, II and III, optionally additionally also of the formulae IV, VI and/or VII.
Besides the compounds of the formulae I, II and III, and optionally additionally of the formulae IV, VI and/or VII, other constituents may also be present, for example in an amount of up to 45% by weight of the mixture as a whole, but preferably up to a maximum of 35% by weight, in particular up to a maximum of 10% by weight.
The other constituents are preferably selected from nematic or nematogenic substances, in particular known substances, from the classes of the azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl cyclohexanecarboxylates, phenylcyclohexanes, cyclohexylbiphenyls, cyclohexylcyclohexanes, cyclohexylnaphthalenes, 1.4-biscyclohexylbiphenyls or cyclohexylpyrimidines, phenyl- or cyclohexyldioxanes, optionally halogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinnamic acids.
The most important compounds which are suitable as constituents of liquid-crystal mixtures of this type can be characterised by the formula VIII
Raxe2x80x94Lxe2x80x94Gxe2x80x94Exe2x80x94Rbxe2x80x83xe2x80x83VIII
in which
L and E are each a carbocyclic or heterocyclic ring system from the group formed by 1.4-disubstituted benzene and cyclohexane rings, 4.4xe2x80x2-disubstituted biphenyl, phenylcyclohexane and cyclohexylcyclohexane systems, 2.5-disubstituted pyrimidine and 1.3-dioxane rings, 2.6-disubstituted naphthalene, di- and tetrahydronaphthalene, quinazoline and tetrahydroquinazoline, 
or a Cxe2x80x94C single bond,
Q is halogen, preferably chlorine, or xe2x80x94CN, and
Ra and Rb are each alkyl, alkoxy, alkanoyloxy or alkoxycarbonyloxy having up to 18, preferably up to 8, carbon atoms, or one of these radicals is alternatively CN, NC, NO2, CF3, F, Cl or Br.
In most of these compounds, Ra and Rb are different from one another, one of these radicals usually being an alkyl or alkoxy group. Other variants of the proposed substituents are also common. Many such substances or also mixtures thereof are also commercially available. All these substances can be prepared by methods known from the literature.
It goes without saying to the person skilled in the art that the ECB mixture according to the invention may also comprise compounds in which, for example, H, N, O, Cl and F have been replaced by the corresponding isotopes.
The construction of the liquid-crystal displays according to the invention corresponds to the usual geometry, as described, for example, in EP-A 0 240 379.
Besides the compounds of the formula I, the mixtures according to the invention preferably comprise one or more compounds of the compounds mentioned below.
The following abbreviations are used: (n, m=1-6; z=1-6) 